A striking signal of dark matter beyond the standard model is the existence of cores in the centre of galaxy clusters. Recent simulations predict that a Brightest Cluster Galaxy (BCG) inside a cored galaxy cluster will exhibit residual wobbling due to previous major mergers, long after the relaxation of the overall cluster. This phenomenon is absent with standard cold dark matter where a cuspy density profile keeps a BCG tightly bound at the centre. We test this hypothesis using cosmological simulations and deep observations of 10 galaxy clusters acting as strong gravitational lenses. Modelling the BCG wobble as a simple harmonic oscillator, we measure the wobble amplitude, Aw, in the BAHAMAS suite of cosmological hydrodynamical simulations, finding an upper limit for the CDM paradigm of Aw<2kpc at the 95% confidence limit. We carry out the same test on the data finding a non-zero amplitude of Aw=11.82+7.3−3.0kpc, with the observations dis-favouring Aw=0 at the 3σ confidence level. This detection of BCG wobbling is evidence for a dark matter core at the heart of galaxy clusters. It also shows that strong lensing models of clusters cannot assume that the BCG is exactly coincident with the large scale halo. While our small sample of galaxy clusters already indicates a non-zero Aw, with larger surveys, e.g. Euclid, we will be able to not only to confirm the effect but also to use it to determine whether or not the wobbling finds its origin in new fundamental physics or astrophysical process.

I explained this in the following way to someone asking about this paper at the Physics Forums:

The really core point, from the abstract is that:

[The] Brightest Cluster Galaxy (BCG) inside a cored galaxy cluster will exhibit residual wobbling due to previous major mergers, long after the relaxation of the overall cluster. This phenomenon is absent with standard cold dark matter where a cuspy density profile keeps a BCG tightly bound at the centre. . . . This detection of BCG wobbling is evidence for a dark matter core at the heart of galaxy clusters.

Ten years ago, this would have been a really big deal since it contradicts the cold dark matter (CDM) hypothesis as an explanation for dark matter phenomena. But, at this point, it is really just piling onto an abundant collection of evidence showing contradictions between the CDM hypothesis and observation.

One of these contradictions (there are several of them) is known as the cusp-core problem, which is that CDM theories, generically, predict that dark matter halos should have a cuspy density profile, but inferences about the distribution of dark matter from the dynamics of visible matter in galaxies and gravitational lensing observations demonstrate that this is not actually the shape of inferred dark matter distributions in galaxies. Instead, inferred dark matter halos distributions have what is known as an "isothermal" distribution of dark matter within the dark matter halo around a galaxy.

So, this result really just confirms in a novel way something that was widely known from other evidence. This is still important, because it makes the conclusion that there really is a cusp-core problem that is not just an artifact of a flaw in some particular methodology that provides the evidence for the cusp-core problem much more robust. But, it doesn't really change the bottom line from existing data.

To prevent a cuspy density profile from emerging in a halo you need some kind of feedback either between dark matter particles or between ordinary matter and dark matter that spreads it out when it gets too dense.

But, that contradicts the assumption made in early cold dark matter theories that dark matter should be collisionless, which has strong support from the failure to direct dark matter detection experiments to see it, from the absence of a strong dark matter annihilation signal, and from the non-detection of dark matter at the Large Hadron Collider (LHC), and is consistent with the success of the lamdaCDM model of cosmology at scales much larger than galaxies and galaxy clusters, although these methods would often miss detect interactions between dark matter and other dark matter that does not result in annihilation of the interacting dark matter particles and take place at relative small distances relative to those important for cosmology.

Warm dark matter proponents have suggested a quantum effect that only kicks in at masses of dark matter particles on the order of 2 keV/c2 or less. Others have proposed self-interacting dark matter (SIDM) models to address the issue. But, those models have their own problems beyond the scope of this discussion.

10 comments:

It's time to look at models which directly violate the Standard Model. One thought is that dark matter is 'sleeping matter' - that ordinary matter becomes dark when the density drops below a certain point. This would imply dark matter has a maximum density (I looked a bit and it seems that it might). Conversely, clouds of normal matter should have a minimum density that is about this same number. (again its seems it might). The whole cuspy problem set is solved in one shot - no cusp as matter is all enlivened in galactic cores. In this model dark matter searches on earth are not going to work.

Its just an idea, but its the kind of thinking that is required - to move through fanciful 'wrong' theories to arrive at the solution. One thing that seems more certain every day is that attacking the problem conservatively from the edges is not going to work.

Would this also contradict (non-materialist) modified gravity hypotheses? Because if DM needs collisions, then it should not work either with modified gravity, which would be smooth for certain values and not "chaotic".

My best guess is that it's actually 5 or 6 parallel universes with their own galaxies and what not, separated by the extra dimensions required for M theory but held together by gravity. So far, other than the issue with no-SUSY that seems to affect M theory at its foundation, I have not seen anything specific that contradicts parallel galaxies and universes originated from the same Big Bang. It could be that M theory is wrong but still right about the need of extra dimensions and that, while those dimensions may be "rolled" locally in our universe are sill "flat" across universes.

What Tom says is that very thinly scattered regular matter would be that DM, ("sleeping matter" is confusing, rather "undetected very thinly spread matter"), am I right? But AFAIK astrophysicists have already rejected that idea based on their measurements: would it be so thinly spread, it'd would exert much less gravitational force than needed, so nope.

As a rather unrelated side note, we should not forget that what causes gravity is not, strictly speaking, matter but ENERGY and that matter is only a "concentrated" form of energy according to Einstein's original formula of m=E/c^2 (that's how he wrote it).

Another possible explanation I have never read about (so I throw here as an open question) is that Dark Energy, which is pressure from empty space (which seems a bit better understood nowadays) is what is causing "dark matter" (unexplained halo-like gravity). This "pressure" ("negative pressure" technically speaking: pushing outwards) causes space to expand but this space would also expand against densely energized space such as galaxies, I presume, and not just "outwards" against the boundaries of the universe. Could dark energy explain dark matter therefore? Would it be consistent with this constraint or rather not?

Just to clarify - by sleeping matter I mean matter that has lost the ability to take part in atomic physics. The atoms get 'lonely' and so as long as you have a low enough density, there is no interaction other than gravity. When they get close to each other they wake up and have orbitals, can absorb light, etc. Another way of putting it is dark matter is some particle that transforms into regular matter as soon as the density gets to some (small) value.

@Tom Andersen: AFAIK your conjecture has long ago be disproved by measurements and anyhow even the most lonely atom, nucleus or electron is affected by electromagnetism at inifite distances (just that at large scales gravity is more dominant). That's why baryonic matter is hot by definition and the supposed components of DM can't be "hot" AFAIK because scientists' data does not allow for it. In other words, as I said before: the density of low density matter is just too low for the effect detected. Also regular matter should behave like, well, regular matter and therefore adopt a flat geometry around the galactic nuclei, just as stars and more dense gas does, i.e. in a disk and not in a spheroid.

@Andrew: very interesting indeed. I get a bit confused because they use the concept of graviton-made Bose-Einstein condensate and, as far as I am concerned, gravitons do not exist (have never ever been demonstrated in any way, remaining heavily conjectural). But I'm willing to concede that the QM-based theorizations around the concept of the graviton are equivalent to the GR-based ones I tend to prefer, in which space-time itself is the "substance" (or "medium" if you prefer) of energy and hence of matter as well.

I tend to think of matter as concentrated or pressed-in space-time, and that's how it actually works per GR. Basically we have two effects: gravity (which does not "really" exists but is mere curving, inwards-pressure of space-time) and "anti-gravity" (dark energy), which is outward pressure of empty space. Somehow they must be one and the same thing and I'm amiss on why would they need the conejctural gravitons at all for the explanation, really. But maybe both concepts converge because both QM and DR are true as far as we can tell and they must converge at some point. This may be it.